1. Field of the Invention
The present invention relates to a permanent magnet type rotary machine and, more particularly, relates to a hybrid (HB) type permanent magnet rotary machine or stepping motor for use in an office automation equipment or the like.
2. Description of the Prior Art
A three-phase permanent magnet type stepping motor is smaller in vibration than a two-phase permanent magnet type stepping motor. A number of transistors used in a bipolar type driving circuit of the three-phase stepping motor is six, whereas a number of transistors used in a bipolar type driving circuit of the two-phase permanent magnet type stepping motor is eight. Accordingly, the three-phase motor is better than the two-phase motor in general viewpoint. However, even in the three-phase motor, a number of harmonics are included in a field magnetic flux formed by the permanent magnet due to a number of pole teeth so that vibrations and noises are generated, even if a number of rotor pole pairs is increased, though the precision of the positioning and the fluctuation of rotation at the low speed of the motor are enhanced. A number multiplied by three, such as 3, 6, 9 or 12 etc. may be considered as a number of stator main magnetic poles of the three-phase motor. As inexpensive three-phase motors, three stator main magnetic poles are used in view of the simplicity in construction. Further, it is preferable to reduce the number of the stator main magnetic poles in order to increase the torque, because an interlinkage magnetic flux per one main magnetic pole is increased if the number of the main magnetic poles is reduced. A total quantity of magnetic flux "PHgr" of a permanent magnet type rotor in case of the three-phase motor having six stator main magnetic poles is the same with that in case of the three-phase motor having three stator main magnetic poles, if the permanent magnet type rotors are the same with each other. It is assumed that a turn number N of the winding for each phase of the motor having six stator main magnetic poles is the same with that of the motor having three stator main magnetic poles, that a torque per one phase, an interlinkage magnetic flux per one stator main magnetic pole, and a turn number of the winding per one stator main magnetic pole of the motor having six stator main magnetic poles are represented by T6, "PHgr"/6 and N/2, respectively, and that a torque per one phase, an interlinkage magnetic flux per one stator main magnetic pole, and a turn number of the winding per one stator main magnetic pole of the motor having three stator main magnetic poles are represented by T3, "PHgr"/3 and N, respectively. In the motor having six stator main magnetic poles, one phase is composed of two stator main magnetic poles, so that following equation (1) can be obtained.
T6=k("PHgr"/6)(N/2)Ixc3x972=k("PHgr"NI/6)xe2x80x83xe2x80x83(1)
Where, I represents a current.
In the motor having three stator main magnetic poles, one phase is composed of one stator main magnetic pole, so that a following equation (2) can be obtained.
T3=k("PHgr"/3)NIxc3x971=k("PHgr"NI/3)xe2x80x83xe2x80x83(2) 
It is apparent from the comparison of the two equations (1) and (2) that the torque of the motor having three stator main magnetic poles is larger than that of the motor having fix stator main magnetic poles.
U.S. Pat. No. 5,289,064 corresponding to the Japanese patent No. 3,140,814 discloses such conventional rotary machine.
In case that the conventional rotary machine having three stator main magnetic poles and a hybrid type (HB) rotor, an unbalance electromagnetic force in the radial direction is generated when the rotary machine is energized, so that harmonic waves are generated and thus a cogging torque is increased if the number of the rotor teeth is large, and that the vibrations and the cogging torque may be increased due to the unbalance electromagnetic force and the eccentricity in the air gap. Accordingly, it is required to improve the rotary machine in the property and the cost.
In the widely used two-phase rotary machine, the number of stator main magnetic pales is eight, and no unbalance electromagnetic force is generated. However, if the number is four, the construction of the rotary machine and the winding work of the winding become simple, so that the cast can be reduced. A torque T8 obtained in case that the number of the stator main magnetic poles is eight can be expressed by an equation (3).
T8=k("PHgr"/8)(N/4)Ixc3x974=k(("PHgr"NI/8)xe2x80x83xe2x80x83(3) 
A torque T4 obtained in case that the number is four can be expressed by an equation (4)
T4=k("PHgr"/4)(N/2)Ixc3x972=k("PHgr"NI/4)xe2x80x83xe2x80x83(4) 
As apparent from the comparison of the two equations (3) and (4), the torque T4 is larger than the torque T8, however, in case that the number is four a radial unbalance electromagnetic force is generated between the stator and the rotor, so that the vibration and noise are increased and that the precision of the positioning is deteriorated.
In the widely used five-phase rotary machine, the number of stator main magnetic poles is ten, and no unbalance electromagnetic force is generated. However, if the number is five, the construction of the rotary machine and the winding work of the winding become simple, so that the cost can be reduced. Further, the torque of the motor having five stator main magnetic poles is larger than the torque of the motor having ten stator main magnetic poles, however, a radial unbalance electromagnetic force is generated between the stator and the rotor, so that the vibration and noise are increased and that the precision of the positioning is deteriorated.
That is, an air gap between the rotor an the stator becomes uneven, because a small air gap between an outer ring of a bearing and a bracket, as well as a small air gap between an inner surface of a bearing and an outer periphery surface of a shaft are varied, so that a cogging torque and a vibration when the motor is energized are increased, and that the noise are generated. The air gap in the permanent magnet type HB rotary machine is normally small as about 50xc3x9710xe2x88x926 m, so that if the air gap becomes uneven by a small fluctuation of the bearing, the vibration and the noise become large. The mechanism to generate the unbalance electromagnetic force will be explained with reference to the conventional example.
U.S. Pat. No. 5,289,064 discloses the three-phase rotary machine having three stator main magnetic poles and a HB type rotor, the number Nr of the rotor teeth being 3nxc2x11, where n is a positive integer. In order to simplify the explanation, the pole pair number of the rotor is determined as one. In such a case that Nr=3nxc2x11, n=0, and Nr=1, the vertically sectional side view of the rotary machine can be shown as FIG. 7, and the relation between the stator and the rotor of the three-phase rotary machine when it is energized ban be shown as FIG. 8 showing a vertical section of the rotary machine. As shown in FIG. 7, a permanent magnet 5 is magnetized in the axial direction of a rotary shaft 4 so as to form two poles. In FIG. 7, a reference numeral 1 denotes three stator main magnetic poles extending radially from an annular magnetic yoke, 2 denotes a HB type magnetic rotor, 3 denotes three-phase stator windings, each wound around each of stator main magnetic poles, 6 and 7 denote front and back brackets of non-magnetic material such as aluminum, respectively, and 8 denotes bearings. The S pole of the rotor 2 is attracted upwards and the N pole of the rotor 2 is attracted downwards, so that the rotary shaft 4 receives a moment force, and if any air gap exists between the rotary shaft 4 and the bearing 8, the air gap between the stator magnetic pole 1 and the rotor 2 becomes uneven. FIG. 9 shows vectors showing components of upper and lower directions of the unbalance force shown in FIG. 8. In FIG. 9, F1 shows an attractive force generated between the U phase of the stator 1 and the S pole of the rotor 2 shown in FIG. 8. The rotor is of HB type, so that the form of the magnetic flux from the permanent magnet is not a sine wave. Accordingly, total attractive forces formed between the N pole of the rotor 2 and the V phase of the stator 1, and between the N pole and W phase of the stator 1 are F2, respectively, if the densities of the magnetic flux in the air gaps of the U, V and W phase are the same substantially. The F1 is equal substantially in value to the F2, but the F1 is opposite in direction to the F2.
FIG. 10A and FIG. 10B show a conventional HB type rotor of the two-phase motor having four stator main magnetic poles. FIG. 10A shows a relation between the stator and the N pole of the rotor, for example. A relation between the rotor small teeth and the four stator main magnetic poles when one phase of the winding is energized is shown in FIG. 11. As shown in FIG. 11, the main magnetic pole {circle around (1)} of one phase is magnetized to S pole, whereas the main magnetic pole {circle around (3)} of one phase is magnetized to N pole. In this case, unbalance forces Fn and Fs are generated as shown in FIG. 10B, a moment force is applied on the bearing as loads, so that the air gap becomes unevn. The affection of the above becomes large in the HB type stepping motor, because the air gap is small.
The above unbalance forces generated in the rotary machine cannot be eliminated by using two sets of HB type rotors.
FIG. 12 shows such a case that a rotor element 21 is magnetized to S pole and a rotor element 22 is magnetized to N pole in each set of HB type rotor. A reference numeral 9 denotes a non-magnetic member inserted between the adjacent rotor elements 21 and 22 of different polarities of adjacent sets of HB type rotors in order to insulate magnetically them. An equation (5) can be obtained if radial forces F1, F2, F3 and F4 are applied to the rotor elements 21, 22, 21, 22 arranged in this order, respectively, a distance between a center of the left side bearing 8 and a point where the radial force F1 is applied is L1, a distance between F1 and F2 is L2, a distance between F2 and F3 is L3, a distance between F3 and F4 is L4, and F1=F2=F3=F4=F, and L2=L4.
xe2x80x83M=L1F1+(L1+L2+L3)F3xe2x88x92(L1+L2)F2xe2x88x92(L1+L2+L3+L4)xc3x97F4=xe2x88x92(L2+L4)Fxe2x80x83xe2x80x83(5)
It is noted from the above that the unbalance moment M is not eliminated.
In order to solve the foregoing problem in the conventional rotary machine, it is an object of the present magnet type rotary machine comprising a stator having an annular magnetic member and three stator magnetic poles extending radially from the annular magnetic member, windings of three phases, each wound around each of the stator magnetic poles, two sets of rotors adjacent each other arranged in the axial direction and faced to the stator with an air gap therebetween, and two permanent magnets magnetized in the axial direction, wherein each of the stator magnetic poles has a plurality of magnetic teeth at the tip end thereof, each set of the rotors consists of two rotor elements, each having Nr pieces of small tooth on the outer peripheral surface thereof, each of two permanent magnets is held by the two rotor elements in each set of the rotor, and the two rotor elements in each set of the rotor are deviated by xc2xd the pitch of the small rotor tooth from each other in the circumferential direction so that the small rotor teeth of adjacent rotor elements of two adjacent sets of the rotors are the same in polarity.
It is another object of the present invention to provide a permanent magnet type rotary machine comprising a stator having an annular magnetic member and four stator magnetic poles extending radially from the annular magnetic member, windings of two phases, each wound around each of the stator magnetic poles, two sets of rotors adjacent each other arranged in the axial direction and faced to the stator with an air gap therebetween, and two permanent magnets magnetized in the axial direction, wherein each of the stator magnetic poles has a plurality of magnetic teeth at the tip end thereof, each set of the rotors consists of two rotor elements, each having Nr pieces of small tooth on the outer peripheral surface thereof, each of two permanent magnets is held by the two rotor elements in each set of the rotor, and the two rotor elements in each set of the rotor are deviated by xc2xd the pitch of the small rotor tooth from each other in the circumferential direction so that the small rotor teeth of adjacent rotor elements of two adjacent sets of the rotors are the same in polarity.
It is further object of the present invention is to provide a permanent magnet type rotary machine comprising a stator having an annular magnetic member and five stator magnetic poles extending radially from the annular magnetic member, windings of five phases, each wound around each of the stator magnetic poles, two sets of rotors adjacent each other arranged in the axial direction and faced to the stator with an air gap therebetween, and two permanent magnets magnetized in the axial direction, wherein each of the stator magnetic poles has a plurality of magnetic teeth at the tip end thereof, each set of the rotors consists of two rotor elements, each having Nr pieces of small tooth on the outer peripheral surface thereof, each of two permanent magnets is held by the two rotor elements in each set of the rotor, and the two rotor elements in each set of the rotor are deviated by xc2xd the pitch of the small rotor tooth from each other in the circumferential direction so that the small rotor teeth of adjacent rotor elements of two adjacent sets of the rotors are the same in polarity.
It is still further object of the present invention is to provide a permanent magnet type rotary machine comprising a stator having an annular magnetic member and six stator magnetic poles extending radially from the annular magnetic member, windings of three phases, each wound around each of the stator magnetic poles, two sets of rotors adjacent each other arranged in the axial direction and faced to the stator with an air gap therebetween, and two permanent magnets magnetized in the axial direction, wherein each of the stator magnetic poles has a plurality of magnetic teeth at the tip end thereof, each set of the rotors consists of two rotor elements, each having Nr pieces of small tooth on the outer peripheral surface thereof, each of two permanent magnets is held by the two rotor elements in each set of the rotor, and the two rotor elements in each set of the rotor are deviated by xc2xd the pitch of the small rotor tooth from each other in the circumferential direction so that the small rotor teeth of adjacent rotor elements of two adjacent sets of the rotors are the same in polarity.
Adjacent end surfaces of the tip ends of the adjacent stator magnetic poles are connected together.
Nr is 3nxc2x11, 4nxc2x11 or 6nxc2x11, where n is a positive integer.
These and other aspects and objects of the present invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following description, while indicating preferred embodiments of the present invention, is given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.